The invention relates to a novel method for selectively etching an integral or unitary cathode substrate and support; and particularly, although not exclusively, to selectively etching a cylindrical one-piece bimetal cathode substrate and support with greater efficiency and at higher yield than previously achieved.
A one-piece bimetal cathode substrate and support sleeve is disclosed by J. C. Turnbull in RCA Technical Notes, TN No. 1159 mailed July 23, 1976. The one-piece part is described generally as a cup-shaped nickel-alloy cathode substrate integrally supported on a thin cylinder and endwall of a structural alloy such as a nickel-chromium alloy. The part is fabricated from a bimetal laminate comprised of layers of the two alloys which are clad or otherwise joined together.
The one-piece bimetal part has been fabricated by the general method disclosed in U.S. Pat. No. 3,432,900 issued Mar. 18, 1969 to D. R. Kerstetter. By this method, a cup-and-sleeve is deep drawn from a bimetal laminate strip. Then, selected portions of the nickel-alloy layer are masked with a temporary coating of an etch-resistant material and then the unmasked portions are etched away with a liquid etchant. In an alternative method disclosed in U.S. Pat. No. 4,441,957 issued Apr. 10, 1984 to W. R. Poff et al., selected portions of the nickel-alloy layer are selectively masked by temporarily pressing surface portions of an etch-resistant, compressible sheet, such as a silicone rubber sheet, against selected surface portions of the part. In both of the prior methods, the unmasked portions of the nickel-alloy layer are etched away either by immersing the part in a bath of liquid etchant or by pumping the liquid etchant into contact with the part.
In a mass-production form of the Poff et al. method, a plurality of cylindrical parts are mounted on separate mandrels with all of the endwalls in a plane, and the endwalls pressed into a common compressible sheet. The mandrels are arranged in an orthogonal array of 25 rows and 12 columns. Two of the centrally located mandrels are replaced by a support member within the chamber housing to provide structural support. The sheet constitutes one wall of a common chamber housing all of the parts. With the chamber filled with liquid etchant, etchant is pumped into, through and out of the chamber. Attempts to mass produce parts by the Poff et al. method, by shortening the etching time and increasing the number of parts in the chamber, were marred by the formation of blemishes and other defects on the etched parts. These defects were believed to be caused by tiny bubbles in the chamber which blocked or retarded the etching in local areas on the surface of the parts. Various attempts to overcome the problem, as by increasing the flow rate of etchant through the chamber or by channeling the flow of etchant in the chamber, reduced the prevalence of some defects but increased the prevalence of other defects.
U.S. Pat. No. 4,376,009 issued on Mar. 8, 1983 to Kunz discloses a limp stream method of etching an integral cathode substrate and support. In a limp stream method of etching, etchant floods the surface of a part except where the surface is masked. The stream is applied intermittently to permit the acid to drain away and to allow gases generated by the chemical reaction to escape when the stream is not impinging. The Kunz method, like the Poff et al. method, utilizes a chamber housing having an orthogonal array of mandrels arranged in 25 rows and 12 columns with two of the mandrels, removed from the center of the array, replaced by a support member. The resulting parts were marred by the presence of blemishes, in particular unetched islands of nickel-alloy, overlying the nickel-chromium structural alloy. It is believed that the incomplete or non-uniform etching of the nickel-alloy layer is caused by stagnant pools of etchant within the chamber housing.